WBI/Britt
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WBI/Britt clear sky chart from 2005-11-05 to 2017-10-31.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
30%
15279
12%
47254
8%
15242
35%
15242
16%
34348

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
16%
898
6%
2678
2%
895
20%
895
9%
2209
January16th to endNight
22%
818
11%
2546
5%
818
27%
818
11%
2015
February1st to 15thNight
20%
702
10%
2287
4%
701
26%
701
11%
1754
February16th to endNight
22%
631
9%
2009
4%
631
27%
631
12%
1540
March1st to 15thNight
22%
720
12%
2012
5%
720
28%
720
16%
1488
March16th to endNight
32%
768
19%
2112
10%
768
39%
768
25%
1422
April1st to 15thNight
23%
674
14%
1791
6%
674
29%
674
19%
1313
April16th to endNight
32%
540
17%
1620
9%
540
38%
540
20%
1121
May1st to 15thNight
39%
477
15%
1557
12%
477
45%
477
19%
1056
May16th to endNight
42%
375
13%
1497
11%
374
52%
374
21%
896
June1st to 15thNight
41%
360
10%
1266
10%
360
47%
360
20%
813
June16th to endNight
46%
360
11%
1260
11%
360
51%
360
24%
794
July1st to 15thNight
54%
340
15%
1190
16%
340
58%
340
32%
663
July16th to endNight
51%
463
14%
1427
14%
434
52%
434
26%
875
August1st to 15thNight
53%
539
16%
1609
13%
536
56%
536
29%
1004
August16th to endNight
49%
577
19%
1723
11%
577
55%
577
29%
1071
September1st to 15thNight
46%
720
16%
1842
8%
720
49%
720
31%
1227
September16th to endNight
46%
717
21%
1983
13%
717
51%
717
29%
1299
October1st to 15thNight
30%
720
15%
2160
10%
720
36%
720
17%
1573
October16th to endNight
20%
768
12%
2345
5%
768
28%
768
12%
1814
November1st to 15thNight
26%
700
14%
2408
11%
700
32%
700
13%
1773
November16th to endNight
19%
720
8%
2520
5%
720
23%
720
8%
2057
December1st to 15thNight
15%
732
5%
2532
2%
732
19%
732
5%
2166
December16th to endNight
14%
960
7%
2880
3%
960
18%
960
7%
2405

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
17%9%36%5%3445%6%34%
48498
Night
17%10%46%5%34446%31%
47419
Cloud CoverDay
17%9%36%5%34446%33%
48508
Night
17%10%46%5%34446%31%
47423
TransparencyDay
49%6%13%20%9%
48182
Night
53%6%11%18%9%
47254
SeeingDay
20%11%30%24%10%3
5996
Night
23%11%28%25%10%1
15283

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
20%17%6%8%6%24346%20%
1259
16th to endDay
17%15%7%9%6%35%336%21%
1677
February1st to 15thDay
12%17%5%8%6%36%446%24%
1575
16th to endDay
15%15%6%9%6%3434426%
1461
March1st to 15thDay
16%10%45%5%36%45%5%32%
1936
16th to endDay
13%8%36%5%25%45%6%38%
2112
April1st to 15thDay
22%8%35%5%24446%32%
2166
16th to endDay
19%8%24313346%40%
2340
May1st to 15thDay
18%8%26%424447%35%
2394
16th to endDay
15%5%2443435%8%42%
2618
June1st to 15thDay
16%6%35%5%35%5%5%8%34%
2691
16th to endDay
14%6%26%5%35%46%8%36%
2700
July1st to 15thDay
11%425%425%46%8%44%
2700
16th to endDay
10%425%4245%6%8%43%
2637
August1st to 15thDay
11%5%25%5%35%45%7%43%
2340
16th to endDay
13%5%25%5%345%5%7%39%
2466
September1st to 15thDay
15%5%25%5%3445%7%38%
2117
16th to endDay
18%5%25%42445%7%39%
1977
October1st to 15thDay
21%7%36%424346%34%
1800
16th to endDay
28%8%47%6%3343424%
1875
November1st to 15thDay
22%11%36%5%34447%27%
1448
16th to endDay
25%15%38%6%3443420%
1440
December1st to 15thDay
20%20%6%8%5%24435%17%
1425
16th to endDay
20%21%6%8%5%3433417%
1344
January1st to 15thNight
21%15%6%8%5%3434420%
2678
16th to endNight
15%16%7%9%7%3434423%
2546
February1st to 15thNight
12%15%6%9%8%34345%24%
2285
16th to endNight
15%14%5%9%7%35%44424%
2009
March1st to 15thNight
17%12%47%5%25%346%30%
2000
16th to endNight
12%8%45%5%24345%41%
2112
April1st to 15thNight
21%10%46%424335%32%
1791
16th to endNight
18%9%35%5%23446%35%
1620
May1st to 15thNight
15%7%35%435%45%7%35%
1557
16th to endNight
12%4245%346%7%7%40%
1495
June1st to 15thNight
15%6%25%5%25%45%7%37%
1266
16th to endNight
12%5%25%5%345%6%8%39%
1260
July1st to 15thNight
10%43432446%8%46%
1260
16th to endNight
10%426%425%448%45%
1527
August1st to 15thNight
12%4245%34447%45%
1620
16th to endNight
12%5%2443445%6%46%
1723
September1st to 15thNight
14%415%4245%46%45%
1842
16th to endNight
14%5%25%423348%43%
1983
October1st to 15thNight
22%6%25%5%34346%33%
2160
16th to endNight
28%9%36%433336%27%
2345
November1st to 15thNight
21%9%36%5%2345%6%30%
2408
16th to endNight
26%15%47%5%2344421%
2520
December1st to 15thNight
21%19%7%8%6%3443317%
2532
16th to endNight
23%18%6%6%6%33345%17%
2880

Percentage Cloud Cover SE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
19%17%7%9%5%23435%20%
1259
16th to endDay
15%17%6%9%6%35%346%21%
1677
February1st to 15thDay
13%16%5%9%7%36%445%24%
1575
16th to endDay
16%16%5%10%5%4433425%
1461
March1st to 15thDay
17%9%46%5%36%445%31%
1947
16th to endDay
13%8%36%6%24447%37%
2111
April1st to 15thDay
21%10%35%5%23447%31%
2166
16th to endDay
20%8%24323345%40%
2340
May1st to 15thDay
19%8%26%5%23447%35%
2394
16th to endDay
15%5%2442445%8%41%
2618
June1st to 15thDay
16%6%36%5%35%5%5%7%35%
2691
16th to endDay
15%5%26%5%35%5%6%8%35%
2700
July1st to 15thDay
11%425%5%35%46%8%42%
2700
16th to endDay
10%5%25%435%5%5%8%42%
2637
August1st to 15thDay
12%5%25%5%35%5%5%7%41%
2340
16th to endDay
14%5%26%5%345%5%7%38%
2466
September1st to 15thDay
15%5%26%5%345%5%7%38%
2117
16th to endDay
20%5%2442435%7%39%
1977
October1st to 15thDay
21%8%36%5%34346%34%
1800
16th to endDay
27%10%37%6%3433423%
1875
November1st to 15thDay
21%12%35%5%3445%6%26%
1448
16th to endDay
24%15%5%8%6%4434319%
1440
December1st to 15thDay
20%21%7%7%5%24435%17%
1425
16th to endDay
19%22%6%9%6%2433416%
1344
January1st to 15thNight
21%16%6%8%5%34445%19%
2678
16th to endNight
15%17%6%9%7%35%33423%
2546
February1st to 15thNight
11%16%6%9%7%34445%23%
2278
16th to endNight
15%14%6%9%7%34445%23%
2009
March1st to 15thNight
17%12%47%5%25%446%29%
2012
16th to endNight
13%7%46%5%24345%42%
2112
April1st to 15thNight
21%10%46%424335%32%
1791
16th to endNight
19%8%36%434446%36%
1620
May1st to 15thNight
15%8%25%5%25%46%8%35%
1556
16th to endNight
12%5%35%5%25%5%6%7%40%
1495
June1st to 15thNight
15%7%25%5%345%5%7%38%
1266
16th to endNight
13%5%345%35%46%10%38%
1260
July1st to 15thNight
10%32443447%9%46%
1260
16th to endNight
10%5%244245%6%7%45%
1527
August1st to 15thNight
11%5%145%345%47%46%
1620
16th to endNight
12%5%25%42345%6%45%
1723
September1st to 15thNight
14%5%24434446%45%
1842
16th to endNight
14%6%245%23347%44%
1983
October1st to 15thNight
21%6%35%5%3445%5%33%
2160
16th to endNight
28%9%36%423335%27%
2345
November1st to 15thNight
20%9%46%5%2345%7%29%
2408
16th to endNight
25%15%46%5%33435%21%
2520
December1st to 15thNight
21%18%6%9%6%3433417%
2532
16th to endNight
23%18%6%7%6%33435%17%
2880

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
65%5%12%12%3
1260
16th to endDay
65%47%13%7%
1677
February1st to 15thDay
60%413%14%5%
1577
16th to endDay
60%310%17%7%
1461
March1st to 15thDay
52%5%16%16%8%
1947
16th to endDay
44%412%21%14%3
2112
April1st to 15thDay
51%5%10%19%11%1
2166
16th to endDay
45%49%23%14%2
2340
May1st to 15thDay
48%5%11%20%13%1
2394
16th to endDay
39%7%14%22%13%1
2622
June1st to 15thDay
45%7%12%22%12%
2691
16th to endDay
44%6%14%22%10%
2700
July1st to 15thDay
34%6%15%28%12%1
2541
16th to endDay
37%6%15%27%11%
2475
August1st to 15thDay
37%6%20%23%11%
2327
16th to endDay
41%7%16%26%7%
2466
September1st to 15thDay
42%6%16%25%7%
2117
16th to endDay
43%6%16%26%6%
1977
October1st to 15thDay
50%6%13%19%10%
1800
16th to endDay
63%6%12%11%5%
1875
November1st to 15thDay
56%8%11%18%4
1448
16th to endDay
70%6%9%9%3
1440
December1st to 15thDay
71%7%9%9%3
1425
16th to endDay
71%49%10%3
1344
January1st to 15thNight
66%6%11%10%4
2678
16th to endNight
64%48%13%7%
2546
February1st to 15thNight
61%5%13%14%5%
2287
16th to endNight
61%413%13%6%
2009
March1st to 15thNight
54%413%18%7%
2012
16th to endNight
43%411%19%16%3
2112
April1st to 15thNight
53%38%19%13%1
1791
16th to endNight
49%48%18%17%2
1620
May1st to 15thNight
46%7%9%20%13%2
1557
16th to endNight
37%8%13%23%15%1
1497
June1st to 15thNight
44%6%12%22%13%1
1266
16th to endNight
39%7%14%23%13%1
1260
July1st to 15thNight
32%5%14%28%16%2
1190
16th to endNight
37%6%15%26%13%
1427
August1st to 15thNight
36%6%17%25%12%
1609
16th to endNight
36%6%16%28%10%1
1723
September1st to 15thNight
38%7%14%26%11%1
1842
16th to endNight
39%6%14%30%8%
1983
October1st to 15thNight
51%7%10%19%11%
2160
16th to endNight
60%5%11%15%6%
2345
November1st to 15thNight
54%8%12%18%5%
2408
16th to endNight
66%7%10%10%3
2520
December1st to 15thNight
72%7%9%8%2
2532
16th to endNight
70%5%9%9%4
2880

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
27%19%34%14%3
61
16th to endDay
37%7%27%21%6%
96
February1st to 15thDay
35%17%37%8%
90
16th to endDay
30%22%29%14%2
81
March1st to 15thDay
26%16%28%22%6%
237
16th to endDay
19%15%31%21%9%2
288
April1st to 15thDay
24%10%33%20%10%1
270
16th to endDay
24%15%29%15%10%4
270
May1st to 15thDay
25%9%26%23%11%3
288
16th to endDay
17%12%32%22%10%3
331
June1st to 15thDay
18%13%32%24%9%1
486
16th to endDay
16%9%38%26%8%2
495
July1st to 15thDay
11%8%34%30%12%3
475
16th to endDay
10%7%31%30%14%4
397
August1st to 15thDay
16%423%30%14%9%
313
16th to endDay
18%8%22%26%15%8%
332
September1st to 15thDay
17%7%22%28%17%6%
315
16th to endDay
20%8%21%28%15%5%
314
October1st to 15thDay
23%9%33%22%8%1
315
16th to endDay
33%16%26%18%31
291
November1st to 15thDay
23%16%34%19%41
67
16th to endDay
31%23%25%18%1
60
December1st to 15thDay
30%20%36%10%3
60
16th to endDay
34%35%23%41
64
January1st to 15thNight
29%13%31%19%41
900
16th to endNight
22%12%31%24%7%
818
February1st to 15thNight
20%16%32%24%5%
704
16th to endNight
21%12%34%25%5%
631
March1st to 15thNight
25%16%29%23%5%
720
16th to endNight
17%10%32%28%10%
768
April1st to 15thNight
25%13%32%22%6%
674
16th to endNight
24%8%28%24%12%1
540
May1st to 15thNight
18%6%27%31%14%1
477
16th to endNight
14%7%30%32%14%
375
June1st to 15thNight
19%7%24%32%15%
360
16th to endNight
16%424%36%16%1
360
July1st to 15thNight
12%323%39%19%1
340
16th to endNight
12%425%33%22%1
463
August1st to 15thNight
13%222%35%19%5%
539
16th to endNight
14%424%29%24%2
577
September1st to 15thNight
15%8%26%26%17%5%
720
16th to endNight
18%7%23%27%19%3
717
October1st to 15thNight
25%8%30%24%10%1
720
16th to endNight
33%14%28%19%3
768
November1st to 15thNight
25%14%27%22%9%
700
16th to endNight
33%14%25%20%6%
720
December1st to 15thNight
32%17%30%15%3
732
16th to endNight
33%13%30%18%3
960
Page generated at 2017-11-01 17:23:40. Version ac04

Copyright 2017 A. Danko


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